The present invention relates to a superconducting quantum interference device for detecting a very small magnetic field, current, voltage, electromagnetic wave and the like and, more particularly, to a superconducting quantum interference device having a high sensitivity and a low noise.
FIG. 7 shows a basic structure of a DC-SQUID which is one type of known superconducting quantum interference device formed using the prior art. A superconductive ring is formed by a washer coil 2 and a pair of Josephson junctions 1 connected to both ends thereof. A bias line 7 for supplying a bias current is connected between the pair of Josephson junctions 1 and to a central portion of the washer coil 2 and is routed to bonding pads 11a. The washer coil 2 includes a slit cover 3 for reducing stray inductance at a slit portion. In this structure, the bias line 7 is routed such that it surrounds the periphery of the DC-SQUID.
The DC-SQUID is a device used for converting a magnetic flux into a voltage whose output varies relative to the magnetic flux crossing a superconductive ring internally at a cycle of one flux quantum ((.PHI..sub.0 : 2.07.times.10.sup.-15 Wb). The higher the modulated voltage, the more sensitive the device.
In the structure shown in FIG. 7, a magnetic flux is produced by the bias current flowing through the bias line 7. The magnetic flux couples to the DC-SQUID itself and also has an influence on the neighborhood thereof. The bias line 7 has significant influence because it forms a large ring.
FIG. 8 shows a configuration diagram of a superconducting quantum interference device wherein two DC-SQUIDs are integrated on a single substrate using the prior art. Bonding pads 11a are arranged in a row on one side of a substrate 12 to facilitate connection to a driving circuit. Each of the two sets of bonding pads 11a are provided close to each other. A bias current is applied to each of the DC-SQUIDs from a bias line 7. In this case, a magnetic flux produced by the bias line 7 of one of the DC-SQUID couples to the other DC-SQUID. The SQUID 1 for detecting an external magnetic flux also detects the magnetic flux produced at the SQUID 2 by the bias current.
FIG. 9 is a view of a SQUID array configured using the prior art. A SQUID array is used for increasing a voltage modulated by DC-SQUIDs and impedance output by the same. A SQUID array is formed by connecting N DC-SQUIDs in series into a row to provide a modulated voltage which is N times that available with one DC-SQUID. In a SQUID array, offset components of magnetic fluxes crossing of the DC-SQUIDs must be nullified or equalized. When offset magnetic fluxes in the superconductive rings are not equal, the periods of magnetic flux-voltage characteristics of the DC-SQUIDs do not match, which makes it impossible to increase the modulated voltage efficiently.
In FIG. 9, the bonding pads hla are provided close to each other taking connection to a driving circuit into consideration. Since the SQUID 1 and SQUID 2 are surrounded by the bias lines 7 in different ways, the bias lines couple different amounts of magnetic flux. Therefore, the offset magnetic fluxes of all of the DC-SQUIDs are different, which disallows matching of magnetic flux-voltage characteristics and hence makes it difficult to increase the modulated voltage efficiently.
In a superconducting quantum interference device according to the prior art, a bias line for supplying a bias current to a DC-SQUID, a SQUID array or the like has been routed so as to surround the periphery thereof taking connection to a driving circuit into consideration. As a result, a bias line forms a large loop which couples an unnecessary magnetic flux into a superconductive ring of a DC-SQUID. There has been another problem in that when a plurality of DC-SQUIDs are integrated on the same substrate, interference occurs between magnetic fluxes produced by the bias currents of each other.
Further, in the case of a SQUID array wherein a plurality of DC-SQUIDs are connected in series, the amount of magnetic flux coupled by the bias lines varies between the DC-SQUIDs. As a result, it has been difficult to increase the modulated voltage efficiently because it has been impossible to match the period of the magnetic flux-voltage characteristics of each of them.